The technological advance in the field of alkali halides electrolysis has brought to an ever diminishing consumption of energy per unity of product. This result is due to the remarkable improvement of the cell geometry design (see for example Italian Application No. 19502 A/80 by the same applicant, as a consequence of both the advent of ion exchange membranes instead of porous diaphragms (see for example British Patent Publication No. 2 064 586 A) and the use of cathodes exhibiting an ever increasing electrocatalytic activity, that is a lower hydrogen overvoltage.
Such cathodes are obtained by applying a ceramic catalytic coating onto a supporting metal substrate, having suitable geometry (for example expanded sheet) and made of a conductive metal, such as nickel, copper and alloys thereof. The ceramic electrocatalytic coating may be directly applied onto the supporting metal substrate by thermal decomposition of liquids containing precursor compounds of the ceramic electrocatalytic materials, either in solution or as dispersions ("paints").
A serious drawback affecting the cathodes thus obtained is represented by the poor adhesion of the coating to the supporting metal substrate due to the substantial structural incompatibility between the oxides film normally formed onto the substrate surface and the ceramic electrocatalytic material of the coating.
Various attempts to solve the above problem have been undertaken. In one case, for example, the coating is applied in repeated layers which have a varying composition, the inner layer being substantially compatible with the supporting metal substrate, and the external one exhibiting a higher electrocatalytic activity (see for example European Patent Publication No. 0129088 A1).
An efficient alternative is represented by a metal interlayer containing ceramic material particles which are isomorphous with the ceramic electrocatalytic material to be thermally deposited, said interlayer being interposed between the substrate and the external coating, at least onto a portion of the metal substrate surface.
Onto said interlayer, having a suitable thickness, a paint is applied, which is constituted by a solution or dispersion of precursor compounds of the ceramic electrocatalytic coating. After removal of the solvent, heating in an oven is carried out at a temperature and for a time sufficient to transform these precursor compounds into the desired ceramic electrocatalytic material. The desired thickness is obtained by repeating the process for the sufficient number of times.
The electrodes thus obtained are used as cathodes for the electrolysis of alkali halides and more particularly for the electrolysis of sodium chloride and to allow for an active lifetime three to eight times longer than conventional cathodes obtained by thermal deposition according to the prior art (see Italian patent Application No. 83633 A/84).
These electrodes further provide for a low overvoltage and a better resistance to poisoning due to heavy metals, such as iron and mercury present in the electrolyte, compared with conventional cathodes, for example cathodes provided with a galvanically deposited, pigmented electrocatalytic coating (see Belgian Pat. No. 848,458 and U.S. Pat. No. 4,465,580).
It is well-known that, in the specific case of brine electrolysis, the impurities more frequently encountered are iron and mercury: iron may come from the use of potassium ferrocyanide as anticaking agent or from corrosion of the ferrous structures of the cathodic compartment or fittings thereof, while mercury is usually present in the brine circuit when the mercury cells are converted to membrane cells.
As soon as these impurities, usually present in the solution under ionic complex form, diffuse to the cathodic surface, they are readily electroprecipitated to their metallic state, thus neutralizing the catalyst active sites.
Catalytic aging, which may depend on various factors such as the type of cathodic material (composition and structure), operating conditions (temperature, catholyte concentration) and the nature of the impurity, may occur remarkably and irreversibly soon after a few hours of operation.
However, the problems affecting durability and efficiency, which involve consequently resistance of the coated surface to poisoning due to metal impurities, are not yet satisfactorily overcome, taking into account the long-term performance required for an industrially efficient cathode.
In fact, while iron concentrations up to 50 ppm do not seem to negatively affect the cathodes potentials of electrodes provided with thermoformed electrocatalytic ceramic material, higher concentrations, up to 100 ppm, being necessary to observe a poisoning effect, in the case of mercury the cathode potential results remarkably increased soon after short periods of time, in the presence of 3-10 ppm of Hg ions.